1. Field of the Invention
This invention relates to thermosetting resins for use as electrical insulating materials and the like.
2. Description of the Prior Art
Among the resins heretofore employed as electrical insulating materials are silicones, polytetrafluoroethylenes, polyimides, epoxy resins, polyesters, phenolic resins, diallyl phthalate resins and so on. The silicones, polytetrafluoroethylenes, polyimides and the like are expensive and hence only used for special purposes. The epoxy resins, phenolic resins, polyesters, diallyl phthalate resins and so on are widely used, but they do not have excellent electrical characteristics or adequate flexibility. From the viewpoint of electrical characteristics, thermoplastic resins, such as, polyethylene, are excellent, but they have poor heat resistance, and hence are limited in use. A resin for use as electrical insulation, especially a resin for impregnation of glass cloth, as in a printed circuit board, or moulding, should have excellent electrical characteristics and also be of a low-viscosity, non-solvent type. Further, resins for printed circuit boards and flexible printed wiring, in addition to having excellent electric characteristics and good impregnating properties, must have good chemical resistance to withstand the process used in fabrication of a printed circuit board and also good mechanical characteristics after being hardened.
Recently, various studies have been conducted on the use for the above purposes of polymers composed mainly of butadiene because of the excellent electrical characteristics of such polymers. The previously available polymers composed principally of butadiene are liquid and of low molecular weight within a very narrow range, and, further, their end groups are terminated with reactive functional groups such as hydroxyl groups, carboxyl groups and the like. However, satisfactory resin compositions for printed wiring or circuit boards have not been obtained up to now. For example, in Japanese Patent Application Publications Nos. 14538/71 and 10672/71, a hydroxyl-terminated butadiene polymer is modified with an .alpha., .beta.-unsaturated acid, or mixed with that acid, and then hardened by copolymerizing with a vinyl monomer in the presence of a suitable peroxide as a hardening agent. However, the hardening speed is low in the case where the hydroxyl-terminated butadiene polymer is modified with the .alpha., .beta.-unsaturated acid, and the copolymerizing is only poorly achieved in the case where the hydroxylterminated butadiene polymer is only mixed with the acid.